Systematic cyclic tests were conducted on 30WGP1600 non-oriented electrical steel under uniaxial stress control. The influence of mean stress, stress amplitude, and peak stress on the material's ratcheting behavior was analyzed. The evolution of dislocation configuration during ratcheting deformation was observed using TEM (Transmission Electron Microscopy), revealing the essence of ratcheting deformation in electrical steel.
The accumulation of irreversible deformation in structural components under cyclic stress is known as the ratcheting effect. This effect can lead to dimensional exceeding or fatigue deformation and failure, resulting in catastrophic consequences. For example, the rotor of an electric vehicle drive motor will experience ratcheting deformation under centrifugal force. Ratcheting deformation increases the probability of contact between the rotor and stator, leading to motor failure. Furthermore, irreversible plastic deformation deteriorates the motor's magnetic properties, severely impairing its function and reducing its service life. Therefore, to optimize motor structure and accurately assess safety, it is essential to fully understand the ratcheting deformation behavior of non-oriented electrical steel used in rotors under high-frequency, low-stress conditions during motor design.
In recent decades, extensive experimental and theoretical research has been conducted domestically and internationally on the ratcheting effect of materials. However, most studies on material ratcheting behavior involve low loading frequencies and high external stresses (greater than the material's yield strength), with the number of cycles within a few thousand. There is a lack of experimental understanding of material behavior under low stress (peak stress less than or equal to yield strength), high loading frequency, and over 100,000 cycles. Research on the ratcheting behavior of non-oriented electrical steel is even less reported. Therefore, this study investigates the evolution of ratcheting strain in 30WGP1600 non-oriented electrical steel under low stress and high cycle numbers, as well as the influence of mean stress, stress amplitude, and peak stress on its ratcheting behavior. The strain change process and the evolution of dislocation structure under low stress are systematically described and analyzed.
This study investigated the ratcheting behavior of 30WGP1600 non-oriented electrical steel under low stress and high cycle numbers, analyzed the ratcheting behavior of the electrical steel, systematically studied the evolution of dislocation configuration during ratcheting deformation, and obtained the following main conclusions.
(1) Regardless of the changes in stress amplitude and mean stress, the ratcheting strain of electrical steel increases with the increase in peak stress. Therefore, peak stress is the dominant factor affecting the ratcheting deformation of electrical steel.
(2) When the applied peak stress is less than 300 MPa, almost no ratcheting deformation occurs, and the electrical steel is in the elastic deformation stage. When the peak stress is greater than 300 MPa but less than 340 MPa, the ratcheting deformation of the electrical steel is small, and the ratcheting saturation state is reached in the initial stage of the cycle. When the peak stress is greater than 340 MPa but less than or equal to the yield strength, the rate of increase in ratcheting strain is initially large and then small, reaching stability after approximately 100,000 cycles.
(3) When the peak stress is between 300 and 340 MPa, the dislocation density is low, and the dislocation configuration is mostly dislocation lines. During reverse unloading, dislocation movement is relatively easy, and the reversibility of deformation is good. When the peak stress is between 340 and 400 MPa, multiple slip systems are easily activated, dislocation density increases, and interaction strengthens, increasing dislocation movement resistance and irreversibility of deformation. The amount of ratcheting deformation increases significantly during this stage.
(4) When the peak stress is the yield strength, in the initial stage of the cycle (i.e., Stage I), the speed of dislocation movement and multiplication is very fast, and the ratcheting strain rate is also high. In Stage II of ratcheting deformation, the dislocation configuration transforms from low-density dislocation entanglement to high-density dislocation walls and primary dislocation cells. Compared with Stage I, the dislocation multiplication rate is lower, and the ratcheting strain rate decreases accordingly. In Stage III of ratcheting deformation, the high-density dislocation configuration evolves into an incomplete dislocation cell structure. The cell structure is relatively stable, and the ratcheting strain rate increases at an almost constant rate.
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